FIG. 1 shows a schematic diagram of a conventional light source driving circuit 100. FIG. 2 shows waveforms 200 illustrating an operation of the light source driving circuit 100 in FIG. 1. FIG. 1 is described in combination with FIG. 2. The light source driving circuit 100 includes a bridge rectifier 104 and a capacitor 116 that covert an AC input voltage from an AC power source 102 to a DC input voltage VIN in order to power a light emitting diode (LED) string 108. The LED string 108 includes multiple LEDs connected series. The light source driving circuit 100 further includes a buck converter 111 including an inductor 118, a diode 106, and a switch 112. The buck converter 111 receives the DC input voltage VIN and provides an output voltage Vo for the LED string 108. A controller 110 monitors a current flowing through the LED string 108 based on a voltage across a resistor 114, and controls a conductance status of the switch 112. If the switch 112 is turned on, a current ILED flows through the LED string 108, the inductor 118, the switch 112 and the resistor 114 to ground. The current ILED increases when the switch 112 is on. If the current ILED increases to a predetermined peak value IPEAK, the controller 110 turns off the switch 112. When the switch 112 is turned off, the current ILED flows through the LED string 108, the inductor 118, and the diode 106. The current ILED decreases when the switch 112 is off. The controller 110 turns the switch 112 on and off alternately. The controller 110 can work in two modes including a constant period mode and a constant off time mode.
In the constant period mode, the controller 110 turns the switch 112 on and off alternately and maintains a cycle period Ts of the switch 112 substantially constant. An average value IAVG of the current ILED can be given by:
                                          I            AVG                    =                                    I              PEAK                        -                                          1                2                            ·                                                                    (                                                                  V                        IN                                            -                                              V                        O                                                              )                                    ×                                                            V                      O                                                              V                      IN                                                        ×                                      T                    S                                                  L                                                    ,                            (        1        )            where VIN is the DC input voltage provided by the bridge rectifier 104 and the capacitor 116, Vo is the output voltage provided by the buck converter (i.e., the voltage across the LED string 108), L is the inductance of the inductor 118, Ts is the cycle period of the switch 112, and IPEAK is the peak value of the current ILED.
In the constant off time mode, the controller 110 turns the switch 112 on and off alternately and maintains an off time TOFF of the switch 112 substantially constant. An average value IAVG of the current ILED can be given by:
                                          I            AVG                    =                                    I              PEAK                        -                                          1                2                            ·                                                                    V                    O                                    ×                                      T                    OFF                                                  L                                                    ,                            (        2        )            where Vo is the output voltage provided by the buck converter 111 (i.e., the voltage across the LED string 108), L is the inductance of the inductor 118, TOFF is the off time of the switch 112, and IPEAK is the peak value of the current ILED.
One of the drawbacks of the light driving circuit 100 is that the average value IAVG of the current ILED varies with the input voltage VIN and the output voltage Vo.
A first situation is that the average value IAVG of the current ILED increases as the input voltage VIN increases. In the constant off time mode, since there may be a system delay between the time when the current ILED increases to the peak value IPEAK and the time when the controller 110 turns off the switch 112, the current ILED will continue to increase for a time period after it reaches the peak value IPEAK. Therefore, if the input voltage VIN increases, the average value IAVG of the current ILED increases accordingly.
A second situation is that the average value IAVG of the current ILED decreases as the input voltage VIN increases. In the constant period mode, according to equation (1), if the input voltage VIN increases, the average value IAVG of the current ILED decreases accordingly.
A third situation is that the average value IAVG of the current ILED decreases as the output voltage Vo increases. In the constant off time mode, according to equation (2), if the output voltage Vo increases, the average value IAVG of the current ILED decreases accordingly.